Synthesis, characterization, and crystal structures of two 6-cobalt-containing dimeric polyoxoanions: [Co2(H2O)10Co4(H2O)2(B-α-XW9O34)2]8− (X = Ge and Si)

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Journal of Molecular Structure 872 (2008) 176–181 www.elsevier.com/locate/molstruc

Synthesis, characterization, and crystal structures of two 6-cobalt-containing dimeric polyoxoanions: [Co2(H2O)10Co4(H2O)2(B-a-XW9O34)2]8 (X = Ge and Si) Zhiming Zhang, Enbo Wang *, Yangguang Li, Haiyan An, Yanfei Qi, Lin Xu Key Laboratory of Polyoxometalate Science of Ministry of Education, Department of Chemistry, Northeast Normal University, Ren Min Street No. 5268, Changchun, Jilin 130024, PR China Received 31 January 2007; accepted 23 February 2007 Available online 12 March 2007

Abstract Two new polyoxotungstates K4Na4[Co2(H2O)10Co4(H2O)2(B-a-GeW9O34)2] Æ 28.5H2O (1) and K2Na6[Co2(H2O)10Co4(H2O)2(B-aSiW9O34)2] Æ 17H2O (2) have been obtained in aqueous solution and characterized by IR, TG, element analysis, magnetism, electrochemistry and single-crystal X-ray analysis. The polyoxoanion frameworks of the two compounds consist of the sandwich-type polyoxoanion [Co4(H2O)2(B-a-XW9O34)2]8 (X = Ge and Si) covalently linked with two Co2+ cations by two l2-O atoms. The magnetic properties of compound 1 have been studied by measuring its magnetic susceptibility in the temperature range 2.0–300.0 K, indicating the existence of intramolecular ferromagnetic Co–Co interactions. The electrochemical properties of the two compounds were detected in pH 3 buffer solutions, and three redox couples were detected in the multi-cobalt-substituted POMs. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Polyoxometalate; Heteropolyoxotungstates; Cobalt; Sandwich-type; Magnetic property; Electrochemical property

1. Introduction Over the past decades, polyoxometalates (POMs) have been attracting extensive interest in the field of solid-state materials chemistry because of their abundant topological properties and their great potential for application in catalysis, photochemistry, ion exchange, electrochromism, magnetism, and electrochemical chemistry [1–3]. Recently, increasing attention has been devoted to the investigation of the transition-metal substituted POMs (TMSPs) [4]. However, the mechanism of formation of the TMSPs has not been known publicly. Therefore, the design and synthesis of the TMSPs are still difficult at a certain extent.

*

Corresponding author. Fax: +86 431 85098787. E-mail addresses: [email protected], [email protected] (E. Wang). 0022-2860/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2007.02.033

Within the class of TMSPs, a great subclass is the sandwich-type polyoxoanions, which accommodate lots of paramagnetic transition metal cations between two lacunary polyoxoanions. The [M4(XW9)2]n/[M4(X2W15)2]n type with two lacunary polyoxoanions connected by a tetrametal-set [5], and [M3(XW9)2]n/[M3(X2W15)2]n type [6] with a trimetal-set between the two lacunary polyoxoanions, are usually reported. Recently, the dinuclear sandwich complexes [M2(XW9)2]n/[M2(X2W15)2]n have been documented [7]. Thus, the introductions of central magnetic and electrochemical active metal-set lead to this class of polyoxoanions exhibit interesting magnetism and electrochemical properties [8,22]. Recently, the single molecule magnets (SMMs) and single-chain magnets (SCMs) have attracted much increasing attention in the whole world because of their great potential for application as molecular units for data storage or quantum computation [9]. We think the sandwich-type polyoxoanions could be an

Z. Zhang et al. / Journal of Molecular Structure 872 (2008) 176–181

excellent choice to get the compounds with features of the SMMs or SCMs. Presently, our group focus on this field and have made great efforts on the synthesis and design of the novel structure POMs containing the paramagnetic transition metal cations, especially for the Fe, Co, Ni, and Mn, with the expect to obtain the complexes with features of the SMMs or SCMs. In our way, first, we want to design and synthesis of the novel structure dimeric POMs with the features of the SMMs and SCMs. Second, the sandwich-type polyoxoanions are used as the building blocks to construct the target compounds. Unluckily, the obtained 6- and 7-nickel-substituted dimeric POMs [10] and the title 6-Co-containing compounds do not possess the anticipant features, but, possessing interesting magnetic properties. We consider this is a feasible way and the further research in this field is underway. Phosphotungstates, silicotungstates, and tungstoarsenates are probably the most intensively studied systems in the above structurally characterized POMs. In contrast with polyoxometalates containing the SiIV, PVand AsV as the heteroatoms, the number of Ge-containing analogues is very small. Up to now, only several dimeric germanotungstates have been reported, such as a-[Ga6(H2O)3 (GeW9O37)2]14 [11], A-b-[Ti6O3(GeW9O37)2]14 [12] and [(UO3) (GeW9O37)2]14 [13]. However, the proposed formulas and structures of all transition-metal-substituted species mentioned above remain to be confirmed by X-ray diffraction. Recently, Kortz et al. reported the structures and the magnetic properties of transition-metal-substituted dimeric germanotungstates, [M4(H2O)2(GeW9O34)2]12 (M = Mn2+, Cu2+, Zn2+, Cd2+), [5d] and, Bi et al. reported the structure, electrochemical and the magnetic properties of hexa-iron(III)-substituted dimeric Ge-containing polyoxoanion [Fe6(OH)3(A-a-GeW9O34(OH)3)2]11 [14]. The synthesis and the structure of a novel mono-cobalt-substituted complex [{Co(H2O)}(l-H2O)2K(Ge2W18O66)] was reported by Liu et al. [15]. Also, the structurally characterized Co-substituted dimeric silicotungstates are scarcely reported previously. Up to now, there are only a few examples of the Co-substituted silicotungstates, the asymmetric dimeric 3-cobalt-substituted [Co3(B-a-SiW9 O33(OH))(B-a-SiW8 O29(OH)2)]11 [16], di-cobalt-containing [{Co(H2O)} {Co(H2O)4}{K(H2O)2}(Si2W18O66)]22 [6e] and the trimeric [Co6(H2O)30{Co9Cl2(OH)3(H2O)9(a-SiW8-O31)3}]5 [4f], documented in the literatures. The multi-cobalt-substituted polyoxoanions mentioned above all exhibit interesting magnetic properties. In this paper, we report the synthesis, characterization and crystal structures of two new polyoxotungstates K4Na4[Co2(H2O)10Co4(H2O)2(B-a-GeW9O34)2] Æ 28.5H2O (1), K2Na6[Co2(H2O)10Co4(H2O)2(B-a-SiW9O34)2] Æ 17H2O (2). Single-crystal X-ray diffraction analysis of the two polyoxoanions reveals a sandwich-type polyoxoanion [Co4(H2O)2(B-a-XW9O34)2] (X = Si and Ge) coordinated to two Co2+ cations complexes. To the best of our knowledge, no previous examples of the sandwich-type polyoxoanion coordinated to transition metal cations have

177

been proposed previously in the Co-substituted POMs chemistry. 2. Experimental section 2.1. Materials and measurements All chemicals were commercially purchased and used without further purification. K8[c-SiW10O36], K8[bSiW11O39], K8[c-GeW10O36], and K8[b-GeW11O39] were synthesized according to the literature [17] and characterized by IR spectra. Elemental analyses (H) were performed on a Perkin-Elmer 2400 CHN elemental analyzer; Si, Ge, W, Co, Na, and K were analyzed on a PLASMA-SPEC(I) ICP atomic emission spectrometer. IR spectra were recorded in the range 400–4000 cm1 on an Alpha Centaurt FT-IR Spectrophotometer using KBr pellets. TG analyses were performed on a Perkin-Elmer TGA7 instrument in flowing N2 with a heating rate of 10 °C min1. Variable-temperature magnetic susceptibility data were obtained on a SQUID magnetometer (Quantum Design, MPMS-7) in the temperature range of 2–300 K with an applied field of 0.1 T. Electrochemical measurements were carried out on a CHI 660A electrochemical workstation at room temperature (25–30 °C) under nitrogen atmosphere. 2.2. Synthesis Synthesis of 1. In a typical synthesis procedure for 1, K8[c-GeW10O36] (0.5 g) and K8[b-GeW11O39] (0.5 g) were dissolved in 30 mL of distilled water with stirring. The insoluble material was removed by filtration. And then, 2 mL of Co(NO3)2 solution (1 M) was added dropwise with vigorously stirring and the mixture was boiled for 2 h. After cooling to room temperature, the filtrate was kept at room temperature, and slow evaporation for two weeks resulting in the brown crystals (Yield 46%). Anal. Calcd for 1(%): K, 2.66; Na, 1.57; H, 1.39; Co, 6.03; Ge, 2.47; W, 56.3. Found: K, 2.56; Na, 1.49; H, 1.42; Co, 6.00; Ge, 2.51; W, 56.1. IR (KBr pellet): 941(m), 889(s), 762(s), 711(s), 537(m), 513(m). Synthesis of 2. The preparation of 2 was similar to that of 1 except that K8[c-SiW10O36] and K8[b-SiW11O39] was used instead of K8[c-GeW10O36] and K8[b-GeW11O39], respectively. Anal. Calcd for 1(%):K, 1.41; Na, 2.49; H, 1.05; Co, 6.03; Si, 1.01; W, 59.7.. Found: K, 1.39; Na, 2.53; H, 1.00; Co, 6.00; Si, 1.03; W, 59.4. IR (KBr pellet): 942(m), 889(s), 760(s), 711(s), 537(m), 513(m). 2.3. X-ray crystallography Single-crystal X-ray data for 1 and 2 was collected on a Rigaku R-AXIS RAPID IP diffractometer equipped with a normal focus 18 KW sealed tube X-ray source (Mo-Ka ˚ ) operating at 50 KV and radiation, k = 0.710, 73 A 200 mA. Data processing was accomplished with the RAX-

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WISH processing program. A numerical absorption correction was applied. The structure was solved by direct methods and refined by full-matrix least-squares on F2 using the SHELXL 97 software [18]. All the non-hydrogen atoms were refined anisotropically. Further details of the X-ray structural analysis are given in Table 1. Selected bond lengths and angles are listed in Table S1 and S2.

3. Results and discussion

tion with the transition-metal cations with expect to get the novel polyoxoanions. When the two compounds were mixed as 1:1, the muticobalt-substituted POMs 1 and 2 were separated by reaction of the mixture with the Co2+ by a one-pot procedure. We have also systemically studied the reaction of the mixture with the other transition-metal cations (such as Cu, Fe, Ni, Mn, Cr) with various conditions, the only polyoxoanion [Cu4(H2O)2(GeW9O34)2]12 which has been reported by Kortz et al. in 2004, was obtained in this reaction system [5d].

3.1. Synthesis

3.2. Structure description

The polyoxoanion [c-SiW10O36]8 obtained by decomposition of the anion [b-SiW11O36]8 usually undergoes an isomerization course in the aqueous solution and was studied frequently recently. The [c-GeW10O36]8 obtained by the decomposition of the anion [b-GeW11O36]8 by Kortz et al. very recently. The structures and the procedures of the synthesis of the two polyoxoanions ([cGeW10O36]8 and [c-SiW10O36]8) were essentially analogous. Accordingly, [b-XW11O36]8 (X = Si and Ge) units also undergo the decomposition course at a certain pH value in the aqueous solution. Inspired by the aforementioned considerations, we have attempted to use the mixture of the two polyoxometalates (([c-GeW10O36]8 and [b-GeW11O36]8, [c-SiW10O36]8 and [b-SiW11O36]8) reac-

Single-crystal X-ray diffraction analyses reveal that the structures of compounds 1 and 2 are very similar. Figs. 1 and 2 are the combined polyhedral/ball-and-stick representations of the polyoxoanions in the two compounds. In each compound, the anion was composed of two isolated Co2+ ions and a sandwich-type polyoxoanion [Co4(H2O)2(B-aXW9O34)2]12 (X = Ge or Si). The two isolated Co2+ ions are covalently linked to the sandwich-type polyoxoanion [Co2(H2O)10Co4(H2O)2(B-a-XW9O34)2]8 by two l2-oxygen atoms resulting in two novel transition-metal-containing POMs. The subtle structural differences between the two polyoxoanions are centered on the l2-oxygen atoms bridging the isolated Co2+ ions and sandwich-type anions. In compound 1, the l2-oxygen atom associated to the atom W(2), and in the compound 2, linked with atom W(4) which is edge-sharing with W(2) atom (see Figs. 1 and 2). The structure of the title polyoxoanions is closed to the Cusubstituted As-containing polyoxoanion [{Cu(b-Ala)2 (H2O)2}2Cu4(H2O)2 (B-a-AsW9O34)2]6 [19]. In the Cusubstituted polyoxoanion, the isolated Cu2+ ions was coordinated with two b-Ala molecules to form the {Co(bAla)2(H2O)2} complex which associated to the sandwich-

Table 1 Crystal data and structure refinement for 1 and 2

Empirical formula Formula mass Temperature (K) ˚) Wavelength (A Crystal system Space group ˚) a (A ˚) b (A ˚) c (A a (°) b (°) c (°) ˚ 3) V (A Z Dcalcd. (g cm3) l (mm1) F0 0 0 Data/restraints/ parameters Goodness-of-fit on F2 R1a [I > 2r(I)] wR2b Largest diff. peak and ˚ 3) hole (e A a b

Compound 1

Compound 2

H81Co6Ge2K4Na4O108.5W18

H58Co6K2Na6O97Si2W18

5874.07 293(2)

5545.66 293(2)

0.71073 A monoclinic P 2(1)/n 18.381(4) 13.622(3) 20.408(4) 90 113.96(3) 90 4669.6(16) 2 4.178 24.064 5254 8085/102/692

0.71073 triclinic P 1 11.743(2) 13.486(3) 16.656(3) 98.57(3) 95.19(3) 113.64(3) 2356.4(8) 1 3.908 23.148 2460 8083/72/631

0.948

1.047

0.0485 0.1279 2.195 and 3.160 e.

0.0421 0.0951 1.660 and 2.148

R1 = RjjFojjFcjj/RjFoj. wR2 = R[w(Fo2Fc2)2]/R[w(Fo2)2]1/2.

Fig. 1. Combined polyhedral/ball-and-stick representation of polyoxoanion 1.

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3.3. Magnetic properties Magnetic susceptibility of compound 1 was investigated in the range 2–300 K at 0.1 T on polycrystalline samples. The vMT versus T plot, shown in Fig. 3, slowly decreases from 25.0 cm3 K mol1 at 300 K to 21.6 cm3 K mol1 at 35 K, then rapidly increases to 25.9 cm3 K mol1 at 7.0 K. Below 7.0 K the product vMT shows a sharp decrease again down to 2 K. Such features agree with the presence of intermolecular antiferromagnetic interactions at higher temperature (above 35 K), and then the strong intramolecular ferromagnetic Co–Co interactions in the Co4 clusters might play the main roles in the temperature range of 7–35 K. Below the 7 K, the sharp decrease may 26 25

12 10

χm-1(mol cm-3)

-1

14

24

3

type polyoxoanion [Cu4(H2O)2(B-a-AsW9O34)2]6 by the weak covalent interaction (the distance of Cu-l2-O is 2.314). As far as we known, this is the only one example of sandwich-type polyoxoanion coordinated to transition metal complexes proposed previously. In polyoxoanions 1 and 2, the [Co4(H2O)2(B-aXW9O34)2]12 (X = Ge and Si) polyoxoanions have the general sandwich-type structure. In polyoxoanion [Co4 (H2O)2(B-a-XW9O34)2]12, the B-type trivacant Keggin clusters B-a-XW9O34, obtained by removing three edgesharing WO6 octahedra from parent Keggin structure [aXW12O40]4, each provides seven oxygen donor atoms (one from the central SiO4 group and one each from the six W atoms) that are capable of coordinating to the central Co4(H2O)2 cluster whose four Co2+ ions lie in the same plane and form a centrosymmetric regular rhomblike cluster to construct the dimeric structure. This structural type had been reported for the first time in 1973 by Weakley et al. for [Co4(H2O)2(B-a-PW9O34)2]10 [20] and the Ascontaining analogues had been described in 2001 by Wang et al. [5g]. This structural type represents one of the largest sandwich-type polyoxoanion classes. The Co-substituted Si and Ge-containing Weakley-type polyoxoanions have not been obtained up to now, although they are all only charge different from that of the P and As-containing analogues and very closely related to the SiIV and GeIV containing polyoxoanions ([M4(H2O)2(B-a-XW9O34)2]12 (X = Si, M = Mn, Cu, Zn; X = Ge, M = Mn, Cu, Zn, Cd)) [5d,5i]. The bond lengths and the angles of the [B-a-XW9O34] units in compounds 1 and 2 are not unusual. The coordination geometry of Co2+ ions in the central set of 1 and 2 is octahedral and the Co–O distances fall into the range of ˚ in 1 and 1.99(3)–2.18(2) A ˚ in 2. In 2.013(10)–2.161(9) A

compound 1, two l2-oxygen atoms O(16) and O(16 A) bridge the sandwich-type polyoxoanion [Co4(H2O)2(B-aGeW9O34)2]12 with Co(1) and Co(1a), respectively, with ˚ , Co(1)–O(16) bond lengths W(2)–O(16) 1.739(9) A ˚ 2.073(10)A and band angles W(2)–O(16)-Co(1) 162.3(8)°; in compound 2, two l2-oxygen atoms O(10) and O(10A), connect the sandwich-type polyoxoanion [Co4(H2O)2(B-aSiW9O34)2]12 with Co(1) and Co(1 A), respectively, with the bond lengths and band angles are following: W(4)˚ , Co(1)-O(16) 2.093(10)A ˚ , W(4)-O(10)O(10) 1.737(10)A Co(1) 137.1(5)°. Thus it can be concluded that the covalent interaction exists between Co(1) and O(16) in 1, Co(1) and O(10) in 2. As shown in Figs. 1 and 2, the Co2+ ions coordinated to the sandwich-type polyoxoanions are also in a six-coordinate environment with the other five sites occupied by five H2O molecules (bond-valence sum calculations have given the proof of that). Furthermore, bond-valence calculations [21] for the two compounds confirm that the cobalt atoms in the central belt of the anion and the isolated Co ions are all in the oxidation state +II, the terminal oxygen atoms associated to the Co2+ ions are all diprotonated.

χT (cm K mol )

Fig. 2. Combined polyhedral/ball-and-stick representation of polyoxoanion 2.

179

23 22

8 6 4 2 0 0

21 0

50

100

150

50

100 150 T (K)

200

200

250

250

300

300

T (K) Fig. 3. The temperature dependence of reciprocal magnetic susceptibility v1 M and the product vMT of a polycrystalline sample of 1 at 0.1 T.

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Fig. 4. Cyclic voltammograms of complex 1 in pH 3 (0.2 M H2SO4+Na2SO4) buffer solution. The working electrode was glassy carbon; the reference electrode was Ag/AgCl. (a) The whole voltammetric diagram at the scan rate of 100 mV/s. (b) Cyclic voltammograms of complex 1 in the pH 3 (0.2 M H2SO4+Na2SO4) buffer solution at different scan rates (from inner to outer: 10, 20, 50, 100, 120, 150, and 200 mV s1).

Fig. 5. Cyclic voltammograms of complex 2 in pH 3 (0.2 M H2SO4+Na2SO4) buffer solution. The working electrode was glassy carbon; the reference electrode was Ag/AgCl. (a) the whole voltammetric diagram at the scan rate of 100mV/s. (b) Cyclic voltammograms of complex 2 in the pH 3 (0.2 M H2SO4+Na2SO4) buffer solution at different scan rates (from inner to outer: 10, 20, 50, 100, 120, 150 and 200 mV s1).

also result from the presence of intermolecular antiferromagnetic interactions. Such magnetic features are similar to the reported compound [Co4(H2O)2(PW9O34)2]10 [22]. Curve fit for 1/vM versus T plot of 1 with Curie–Weiss law got the result h  4.33 K in the temperature range of 35–300 K (Fig. 3), in accordance with the intermolecular antiferromagnetic interactions at higher temperature (above 35 K). Because two more Co centers were introduced into the compound, no suitable model was found to fit the susceptibility for the moment. More detailed magnetism research of 1 is underway. 3.4. Electrochemistry The electrochemical properties of compound 1 was detected in pH 3 (0.2 M H2SO4+Na2SO4) buffer solution at the scan rate of 100 mV s1. In the potential domain explored, three redox peaks appear and the mean peak potentials E1/2 = (Epa + Epc)/2 are 0.431 V (I-I 0 ), 0.697 V (II–II 0 ) and 0.978 V (III–III 0 ) (vs. Ag/AgCl), respectively (Fig. 4a). The two reversible peaks II–II 0 and III-III 0 correspond to the redox of the WVI in the polyoxo-

anion framework and the domain where the two waves located at was also observed in the other sandwich-type POMs [16,6f]. At scan rates lower than 200 mV s1, the peak currents of the peak II–II 0 were proportional to the scan rate, which indicates that the redox process of 1 is surface-controlled (see Fig. 4b).The first one redox peaks (I–I 0 ) is associated to the redox processes of the Co2+ centers. Controlled potential electrolysis in pH 3 buffer solution was carried out at 0.10v, no characteristic blue color of the solution was observed in this system, which also could give the proof of that the redox waves located at the positive domain corresponding to the redox of the cobalt ions in the polyoxoanions. As the compound 2 is structurally analogous with 1, a similar electrochemical behavior with only slightly different formal potentials was also observed for compound 2 (see Fig. 5). 4. Conclusions In conclusion, we have successfully synthesized two new dimeric polyoxotungstates K4Na4[Co2(H2O)10Co4(H2O)2 (B-a-GeW9O34)2] Æ 28.5 H2O (1) and K2Na6[Co2(H2O)10

Z. Zhang et al. / Journal of Molecular Structure 872 (2008) 176–181

Co4(H2O)2(B-a-SiW9O34)2] Æ 17 H2O (2) in an aqueous solution system. The polyoxoanion frameworks of the two compounds consist of the sandwich-type polyoxoanion [Co4(H2O)2(B-a-XW9O34)2]8 convalently linked with two Co2+ cations by two l2-O atoms. The measurement of magnetic properties of compound 1 reveals that the existence of intramolecular ferromagnetic Co–Co interactions in polyoxoanion 1. The electrochemical properties of the two compounds were detected in the pH 3 buffer solutions and three redox couples were detected in the multi-cobaltsubstituted POMs. Future research will focus on attempting to study the detailed magnetic properties of the two compounds and design and synthesis of the POMs-containing complexes with features of the SMMs or SCMs.

[6]

Acknowledgment The authors thank the National Natural Science Foundation of China (20371011) for financial support. Appendix A. Supplementary data [7]

TG curves and IR spectra and additional tables and figures, X-ray crystallographic information files (CIF) are available for compounds 1 and 2. Further details on the crystal structure investigations may be obtained from the Fachinformationszentrum Karlsruhe, 76344 EggensteinLeopoldshafen, Germany (fax: +49 7247 808 666; e-mail: crysdata@fiz-karlsruhe.de), on quoting the depository numbers CSD-417596 for 1, CSD-417597 for 2. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.molstruc.2007.02.033.

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